I.10 Errors in surveys. A Gallup Poll found that 46% of American smartphone users agree with the statement, “I can’t imagine my life without my smartphone.” The Gallup press release says:

Results of attitudes and behaviors of smartphone usage are based on 15,747 members of the Gallup Panel who have smartphones. . . . For results based on this sample, one can say that the margin of sampling error is ±1 percentage point at the 95% confidence level.

The release also points out that this margin of error is due only to sampling error. Give one example of a source of error in the poll result that is not included in this margin of error.